In fiber optical telecommunication systems, a problem that can rise when an optical signal generated by a laser optical source is transmitted along a fiber is the Stimulated Brillouin Scattering (SBS). The stimulated Brillouin scattering is a known inelastic process of interaction between acoustic and optical waves that propagate in the fiber, made possible by nonlinear effects of the transmissive medium. The thermally excited acoustic waves (phonons) produce a periodic modulation of the index of refraction due to electrostriction. The SBS causes the back-reflection of part of the light that propagates inside the fiber and a contemporary reduction of its frequency (Brillouin shift). The decrease of the light frequency is of the order of 10-20 GHz for silica fibers.
The stimulated Brillouin scattering, as a matter of fact, limits the maximum optical power that can be exploited to transmit signals, since when a certain threshold (hereinafter also referred to as SBS threshold) of optical power is exceeded, the greatest part of the optical power above the threshold is reflected back towards the transmission apparatus. The portion of reflected light, in addition to reducing the power transmitted in the fiber, returns to the transmitter degrading the optical system performances.
Different characteristics of the optical system define the SBS power threshold, such as the wavelength of the signal and the characteristics of the transmission optical fiber employed, for instance its effective area value, the material and the doping profile thereof.
Typically, the laser sources for telecommunications do not emit a monochrome radiation, in the sense that the signal emitted by a non-modulated laser has a finite line width. Additionally, the modulation operated on the optical carrier with the purpose of conveying the useful signal in the transmissive tends to further widen the line width of the emitted power (generally defined as optical power per unit of frequency or optical wavelength). Typical values of line width of the output signal of an external cavity laser for optical systems are comprised between 10 and 100 MHz.
A method to reduce the SBS requires the artificial increase of the power spectral density of a transmitting laser and therefore of the spectral emission width so as to reduce the levels of average optical power for unit frequency.
Recently the interest in having tunable optical laser sources has increased, especially to be used as transmitters in wavelength division multiplexing (shortened, WDM) systems and high channels density WDM systems, the DWDMs (dense wavelengths division multiplexing), in which a plurality of separate data flows are simultaneously transmitted in a single optical fiber and every channel is generated by modulating light of suitable frequency or wavelength emitted by a laser. Additionally, the tunable lasers can be for instance used in virtual private networks based on wavelength addressing.
A technological solution widely used with the purpose of achieving its operation on single longitudinal mode and to guarantee the spectral purity and frequency stability required by most of the applications, is that of the configuration external cavity that offers a good flexibility, because the optimization of the laser parameters can be entrusted to a suitable choice of the typology, of the number and of the related specifications of the different optical elements that can be inserted in the laser cavity. Moreover, high output powers are generally easily obtainable.
The selection of the wavelength or frequency of the output signal from an External Cavity Laser (ECL) is generally accomplished using a tunable filter by means of various mechanisms, like for instance a thermo-optical, electro-optical or piezo-electrical mechanism.